Some Aerodynamic Considerations in the Design of Centrifugal Compressor Impellers

Abstract

A simple one dimensional approach to determine the size of the centrifugal impeller for maximum through flow has been indicated. This design procedure does not mean that the centrifugal impeller could be finally decided on the results obtained from this article. It must be borne in mind that this only deals with some aspects of the design and in itself is not a complete one by which the impeller could have its final shape and size. The major idea of introducing this paper is to highlight the importance of some of the parameters that go towards influencing the design of the centrifugal impeller. The stage pressure, temperature and work output of a centrifugal machine is basically dependant on the major dimensions like the diameter of the impeller ‘d2’ and its tip speed ′u2 ‘, the vane outlet angle ‘β2’ and the flow coefficient ‘∅2’ at exit ‘c2m/ u2’. Further, the influence of the inlet vane angle ‘β1’ and the inlet Mach number ‘M1’of the flow seriously affect the design of the high speed centrifugal impeller. Some guidelines have been indicated. The shape of the impeller channel passages are to be designed based on whether the compressor needs to have an energy build up in the impeller or a high pressure generation within the impeller of the compressor. In such cases, the radial diffuser at the exit of the impeller plays a very crucial role. Since the speed at which the impeller rotates is of interest, it is absolutely necessary for the designer to pay attention to the stress calculations of the impeller rear and front shrouds. Nomenclature: α fluid angle β vane, angle δ semi cone angle, boundary layer thickness ∅ flow coefficient cm /u2 ρ fluid density σ Slip fctor א , k ratio of specific heats cp /cv ω angular velocityType equation here. a, A constants, annulus area, area, velocity of sound b, bl, B vane height, blade c absolute velocity cp Specific heat at constant pressure cv Specific heat at constant volumeא h vane height k ratio of specific heats cp /cv m, M mas flow, Mach no. n distance normal to streamline p static pressure r, R radius t1, t ,T time, temperature V volume flow rate w,W relative velocity, Specific work X coordinate direction, linear length y, Y coordinate direction z coordinate direction, number of vanes Subscripts _ average ‘ first differentiation with reference to x Some Aerodynamic Considerations in the Design of Centrifugal Compressor Impellers www.iosrjournals.org 2 | Page “ second differentiation with reference to x bl blade h hub m meridional component 0 total r relative component t tip u tangential component 1 inlet of impeller 2 exit of impeller